High temperature calcium silicate insulation

ABSTRACT

A calcium silicate insulation product includes a calcium silicate hydrate matrix that is predominantly but not substantially all xonotlite, for example 51-90 weight percent xonotlite and 10-49 weight percent tobermorite. The insulation product also contains wollastonite, and may include fines of lime and silica and reinforcing carbon fibers. A method of producing the insulation product includes providing the components of the insulation as dry solids, blending the dry solids with water to form a slurry, filter pressing the slurry to form a pressed shape, and curing the pressed shape under steam pressure only until the desired proportions of xonotlite and tobermorite are achieved in the calcium silicate hydrate matrix.

This application is a continuation of U.S. patent application Ser. No.14/688,304 filed Apr. 16, 2015, now U.S. Pat. No. 9,670,098 issued Jun.6, 2017, and titled “High Temperature Calcium Silicate Insulation”, theentire disclosure of which is hereby incorporated by reference hereinfor all purposes.

BACKGROUND OF THE INVENTION

Calcium silicate insulation is often used in extreme temperatureenvironments, due to its good insulating properties, durability, fireresistance, machinability, and inertness. For example, in the aluminumfoundry industry, calcium silicate plates are often used as dies for theforming of aluminum ingots, as well as for creating troughs and the likefor the channeling of molten aluminum. Thus, the insulation is in longterm direct contact with the molten aluminum.

Such harsh conditions, and especially temperature fluctuations thatoccur under repeated use cycles of the insulation, can cause cracking orother failures of the components made of calcium silicate, necessitatingtheir regular replacement. There is accordingly a need for even moredurable calcium silicate insulation, for increased part life anddecreased frequency of part replacement.

BRIEF SUMMARY OF THE INVENTION

According to one aspect, an insulating material comprises a calciumsilicate hydrate matrix having 51-90 weight percent xonotlite and 10-49weight percent tobermorite. The insulating material further compriseswollastonite, and the weight of wollastonite in the insulating materialis 25-200 percent of the combined weights of the xonotlite andtobermorite. The insulating material is formed into a substantiallyuniform rigid body. In some embodiments, the calcium silicate hydratematrix has 62-81 weight percent xonotlite and 19-38 weight percenttobermorite. In some embodiments, the calcium silicate hydrate matrixhas 66-76 weight percent xonotlite and 24-34 weight percent tobermorite.In some embodiments, the insulating material further comprises carbonreinforcing fibers, and the insulating material has a flexural strengthof at least 1300 lb/in². In some embodiments, the insulating materialhas a flexural strength of at least 1400 lb/in². In some embodiments,the insulating material has a flexural strength of at least 1500 lb/in².In some embodiments, the insulating material has a compressive strengthof at least 2600 lb/in². In some embodiments, the insulating materialhas a compressive strength of at least 3200 lb/in². In some embodiments,the density of the insulating material is less than 55 lb/ft³. In someembodiments, the insulating material has a linear shrinkage in length ofless than 0.5 percent at 1380° F. In some embodiments, the insulatingmaterial has a linear shrinkage in length of less than 0.4 percent at1380° F. In some embodiments, the insulating material has a linearshrinkage in thickness of less than 0.90 percent at 1380° F.

In some embodiments, the insulating material further comprises carbonfibers and fines of lime and silica, and the insulating material has aflexural strength of at least 1300 lb/in², a compressive strength of atleast 2600 lb/in², and a density of less than 55 lb/ft³. In someembodiments, the insulating material further comprises carbon fibers andfines of lime and silica, and the insulating material has a flexuralstrength of at least 1500 lb/in², a compressive strength of at least3200 lb/in², and a density of less than 55 lb/ft³. In some embodiments,the weight of wollastonite in the insulating material is 50-175 percentof the combined weights of the xonotlite and tobermorite. In someembodiments, the weight of wollastonite in the insulating material is120-160 percent of the combined weights of the xonotlite andtobermorite.

According to another aspect, a method of making an insulating materialcomprises providing as dry solids 10-20 weight percentpreviously-prepared xonotlite, 15-25 weight percent lime, 15-25 weightpercent silica, and 35-55 weight percent wollastonite. The methodfurther comprises blending the dry solids with water to form a slurry,filter pressing the slurry to form a pressed shape; and curing theresulting pressed shape under steam pressure only until the resultingcomposition includes, in addition to the wollastonite and any residuallime and silica, a calcium silicate hydrate matrix having 51-90 weightpercent xonotlite and 10-49 weight percent tobermorite. In someembodiments, the method comprises curing the resulting pressed shapeunder steam pressure only until the resulting composition includes, inaddition to the wollastonite and any residual lime and silica, a calciumsilicate hydrate matrix having 62-81 weight percent xonotlite and 19-38weight percent tobermorite. In some embodiments, the method comprisescuring the resulting pressed shape under steam pressure only until theresulting composition includes, in addition to the wollastonite and anyresidual lime and silica, a calcium silicate hydrate matrix having 66-76weight percent xonotlite and 24-34 weight percent tobermorite. In someembodiments, curing the resulting pressed shape under steam pressurecomprises curing the resulting pressed shape under saturated steam at12-18 bar for 5 to 8 hours.

DETAILED DESCRIPTION OF THE INVENTION

The durability of calcium silicate insulation is related especially tothe flexural and compressive strength of the insulation, as well as tothe shrinkage and expansion of the insulation under changes intemperature. More shrinkage and expansion tends to promote cracking,splintering, and eventual degradation of the insulation, and thus, lowershrinkage is desirable. In addition, higher mechanical strength promotesdurability, and a material with a higher mechanical strength tends toresist cracking and splintering better than a material with a lowermechanical strength.

The properties of the material are in turn determined by the particularproportions of the constituents combined to make the material, and theprocessing used in making the final material from the constituents.

Calcium silicate insulation according to embodiments of the inventionincludes combinations of different calcium-silicate compounds andcrystalline structures, for example xonotlite, tobermorite, andwollastonite. In particular, xonotlite and tobermorite are calciumsilicate hydrates having different crystalline structures, and theircombination is sometimes referred to as a calcium silicate hydratematrix. Xonotlite is a monohydrate (6CaO.6SiO2.H2O), while tobermoritecan be up to a penta-hydrate (5CaO.6SiO2.5H2O). By contrast,wollastonite is non-hydrated, and may be considered an inert filler inthe material composition.

In general, the shrinkage of an insulation material increases with thehydrate content, so any tobermorite contributes disproportionately toshrinkage as compared with an equal amount of xonotlite. In turn,wollastonite shrinks comparatively little as compared with xonotlite andespecially as compared with tobermorite. However, the presence ofsignificant amounts of tobermorite may contribute to the integrity andstrength of the insulation material. The material strength anddurability can be further improved by the addition of reinforcingfibers, for example carbon fibers.

Prior calcium silicate materials have tended to include a majority oftobermorite or substantially all xonotlite in the calcium silicatehydrate matrix. Embodiments of the invention combine xonotlite andtobermorite with wollastonite and other components to achieve improvedperformance in strength and shrinkage, and consequent improvements inthe durability of the insulation material.

In particular, a calcium silicate insulation according to embodimentshas a calcium silicate hydrate matrix that is predominantly (more than50 weight percent) xonotlite, but is not substantially all xonotlite.That is, the calcium silicate hydrate matrix includes significanttobermorite as well, for example 10 weight percent or more. Thisrelationship may also be expressed as the ratio of xonotlite totobermorite by weight in the finished insulation product. For example,equal proportions of xonotlite and tobermorite in the calcium silicatehydrate matrix would result in a xonotlite:tobermorite weight ratio of1:1 in the finished product. Similarly, a calcium silicate hydratematrix having 70 weight percent xonotlite and 30 weight percenttobermorite results in a xonotlite:tobermorite weight ratio of 2.33 inthe finished insulation product.

According to other embodiments, the proportion of xonotlite andtobermorite (the calcium silicate hydrate matrix) to the amount ofwollastonite in the insulation may be specified. The presence ofwollastonite in significant quantities may tend to limit the shrinkageof the insulation product, and therefore may also contribute to thedurability of the insulation product. For example, the weight ofwollastonite in the finished product may be 25-200 percent of thecombined weights of xonotlite and tobermorite, and preferably 50-175percent, and more preferably 120-160 percent. In some embodiments,wollastonite may make up 25-65 percent by weight of the mineral contentof the finished product (excluding any reinforcing fibers), preferably30-60 percent, and more preferably 40-50 percent.

Finally, the insulation product contains fines of lime and silica, whichmay be residual from the process of forming the insulation product as isdescribed in more detail below. The fines may make up 1-20 weightpercent of the mineral content of the finished product (excluding thereinforcing fibers), preferably 2-15 percent, and more preferably 5-12percent.

While the mechanical strength of calcium silicate insulation typicallyincreases with density, insulation products according to embodiments ofthe invention may achieve high mechanical strength without excessivedensity. For example, in one particular embodiment, the product has aflexural strength greater than 1500 lb/in² and a compressive strengthgreater than 3200 lb/in² (as measured according to ASTM test standardsC203 and C165), but a density of less than 55 lb/ft³.

Insulation products such as those described above may be produced byblending selected dry solids in selected proportions with water to forma slurry, filter pressing the slurry, and curing the resulting pressedshape.

In embodiments, the combined dry solids may include:

10-20 weight percent previously-prepared xonotlite, for example 15weight percent;

15-25 weight percent lime, for example 19 weight percent;

15-25 weight percent silica, for example 19 weight percent;

35-55 weight percent wollastonite, for example 45 weight percent; and

1-5 weight percent carbon fibers, for example 2 weight percent.

The previously-prepared xonotlite may be supplied in slurry, powder,granular, or another suitable form. The lime and silica may be providedin any suitable form, but are preferably supplied as fine powder or“flour”. Similarly, the wollastonite may be provided in any suitableform, such as a powder or granular form. These components are generallyreadily commercially available.

The carbon fibers may be, for example, Tenax®-A HT C124 fibers 6 mm longand having a diameter of 7 microns, available from Toho Tenax America,Inc. of Rockwood, Tenn., USA, or another suitable kind of fibers. Forexample, other suitable lengths and diameters of fibers may be used. Thefibers may include a sizing to promote their dispersion in water.

The combined dry solids are then blended with water to form a slurry.Any suitable amount of water may be used, for example 4-6 pounds ofwater per pound of the combined dry solids. In one example embodiment,4.5 pounds of water is used per pound of the dry solids. The blendingpreferably utilizes continuous high-shear mixing, but in someembodiments, other kinds of mixing may be used, for example intermittentmixing. The slurry is preferably maintained at a temperature thatpromotes its workability and filtration. In some embodiments, the slurrymay be at about 95-135° F. for blending.

The slurry is then filter pressed to remove excess water, resulting in apressed shape such as a slab, board, or other shape.

The resulting shape is then cured at elevated temperature and pressure,for example by autoclaving in saturated steam at a pressure of 12-18bar, for example 15 bar. During the curing, tobermorite and xonotliteare formed from the lime and silica. While tobermorite can be formed atlower curing pressures, for example 10 bar, the formation of xonotliterequires higher pressures.

The transformation of lime and silica to tobermorite and xonotlite isalso affected by the amount of time that the product is cured. Thelonger the curing time (at pressures sufficient to form xonotlite), themore xonotlite is formed. According to embodiments of the invention, thecuring is continued only until the desired proportions of xonotlite andtobermorite are achieved in the calcium silicate hydrate matrix, forexample 51-90 weight percent xonotlite and 10-49 weight percenttobermorite. The curing is not continued beyond this time, in order toavoid the creating of a calcium silicate hydrate matrix that issubstantially all xonotlite. As is shown below, such a product would nothave the desired performance properties. In embodiments, the curing timemay be, for example, 5-8 hours, depending on the steam pressure. Theproduct is preferably held under light pressure between wire racksduring the curing cycle.

It is believed that the surprising strength and durability of insulationproducts according to embodiments of the invention result primarily fromthe particular proportions of xonotlite, tobermorite, and wollastonite,in addition to the effect of the reinforcing carbon fibers.

It will be recognized that the conversion of lime and silica to otherforms may not be complete at the time the desired proportions ofxonotlite and tobermorite are achieved. Accordingly, the resultinginsulation product may include some residual fines of lime and silica.

The cured insulation product may be further dried and heat treated, forexample in an oven at temperatures between 200° F. and 700° F., over aperiod of days. The product may then be sanded to final shape.

EXAMPLES

Table 1 below gives the mineral content of several example materialcompositions A-F of insulation products, and Table 2 gives variousperformance parameters of the example compositions A-F. Each of theexamples also includes about 1.75-2.0 weight % carbon fibers, notincluded in the mineral compositions in Table 1.

TABLE 1 Xonotlite Xonotlite/ Density Xonotlite Tobermorite WollastoniteFines % in calsil Tobermorite lb/ft³ wt. % wt. % wt. % wt. % matrixRatio A 51.7 38 24 34 4 61 1.56 B 51.4 20 22 44 14 48 0.92 C 52.4 27 1154 8 71 2.44 D 61.1 45 11 39 4 80 4.00 E 52.2 35 25 38 2 58 1.38 F 52.763 3 34 1 95 19.00

TABLE 2 Flexural Compressive Strength Strength Hardness Shrinkage at1380° F. lb/in² lb/in² Rockwell D % length/thickness A 1061 2366 68/700.3/0.98 B 950 1410 68/70 0.5/0.95 C 1549 3262 72/73 0.36/0.77  D 15903641 75/80 0.3/1.05 E 1062 2345 F 1340 2649 76/66 0.3/1.40

For the purposes of this disclosure, flexural strength is as measuredaccording to ASTM test standard C203, and compressive strength is asmeasured according to ASTM test standard C165. Shrinkage is as measuredaccording to ASTM test standard C356. For the purposes of thisdisclosure, shrinkage in “thickness” means in the smallest dimension ofthe tested shape, for example the thickness of a slab or board shape,and shrinkage in “length” means in dimensions other than the smallestdimension, for example the length or width of a slab or board shape.

Examples B and F are comparative examples. Example B has less than 50%xonotlite in the calcium silicate hydrate matrix and more than 50%tobermorite. As can be seen, Example B is relatively weak and has arelatively large shrinkage as compared with some embodiments of theinvention. Example F has a very large proportion of xonotlite in thecalcium silicate hydrate matrix, and also does not achieve maximumstrength, and has a high shrinkage rate.

Having described several embodiments, it will be recognized by those ofskill in the art that various modifications, alternative constructions,and equivalents may be used without departing from the spirit of theinvention. Additionally, a number of well-known processes and elementshave not been described in order to avoid unnecessarily obscuring thepresent invention. Accordingly, the above description should not betaken as limiting the scope of the invention.

Where a range of values is provided, it is understood that eachintervening value, to the tenth of the unit of the lower limit unlessthe context clearly dictates otherwise, between the upper and lowerlimits of that range is also specifically disclosed. Each smaller rangebetween any stated value or intervening value in a stated range and anyother stated or intervening value in that stated range is encompassed.The upper and lower limits of these smaller ranges may independently beincluded or excluded in the range, and each range where either, neitheror both limits are included in the smaller ranges is also encompassedwithin the invention, subject to any specifically excluded limit in thestated range. Where the stated range includes one or both of the limits,ranges excluding either or both of those included limits are alsoincluded.

As used herein and in the appended claims, the singular forms “a”, “an”,and “the” include plural referents unless the context clearly dictatesotherwise. Thus, for example, reference to “a process” includes aplurality of such processes and reference to “the device” includesreference to one or more devices and equivalents thereof known to thoseskilled in the art, and so forth.

Also, the words “comprise,” “comprising,” “include,” “including,” and“includes” when used in this specification and in the following claimsare intended to specify the presence of stated features, integers,components, or steps, but they do not preclude the presence or additionof one or more other features, integers, components, steps, acts, orgroups.

What is claimed is:
 1. An insulating material, comprising: a calciumsilicate hydrate matrix having 51-90 weight percent xonotlite and 10-49weight percent tobermorite; and wollastonite; wherein the weight ofwollastonite in the insulating material is 25-200 percent of thecombined weights of the xonotlite and tobermorite; and wherein theinsulating material is formed into a substantially uniform rigid body.2. The insulating material of claim 1, wherein the calcium silicatehydrate matrix has 62-81 weight percent xonotlite and 19-38 weightpercent tobermorite.
 3. The insulating material of claim 1, wherein thecalcium silicate hydrate matrix has 66-76 weight percent xonotlite and24-34 weight percent tobermorite.
 4. The insulating material of claim 1,further comprising carbon reinforcing fibers, wherein the insulatingmaterial has a flexural strength of at least 1300 lb/in².
 5. Theinsulating material of claim 4, wherein the insulating material has aflexural strength of at least 1400 lb/in².
 6. The insulating material ofclaim 4, wherein the insulating material has a flexural strength of atleast 1500 lb/in².
 7. The insulating material of claim 1, wherein theinsulating material has a compressive strength of at least 2600 lb/in².8. The insulating material of claim 1, wherein the insulating materialhas a compressive strength of at least 3200 lb/in².
 9. The insulatingmaterial of claim 1, wherein the density of the insulating material isless than 55 lb/ft³.
 10. The insulating material of claim 1, wherein theinsulating material has a linear shrinkage in length of less than 0.5percent at 1380° F.
 11. The insulating material of claim 1, wherein theinsulating material has a linear shrinkage in length of less than 0.4percent at 1380° F.
 12. The insulating material of claim 11, wherein theinsulating material has a linear shrinkage in thickness of less than0.90 percent at 1380° F.
 13. The insulating material of claim 1, furthercomprising carbon fibers and fines of lime and silica, wherein theinsulating material has: a flexural strength of at least 1300 lb/in²; acompressive strength of at least 2600 lb/in²; and a density of less than55 lb/ft³.
 14. The insulating material of claim 1, further comprisingcarbon fibers and fines of lime and silica, wherein the insulatingmaterial has: a flexural strength of at least 1500 lb/in²; a compressivestrength of at least 3200 lb/in²; and a density of less than 55 lb/ft³.15. The insulating material of claim 1, wherein the weight ofwollastonite in the insulating material is 50-175 percent of thecombined weights of the xonotlite and tobermorite.
 16. The insulatingmaterial of claim 1, wherein the weight of wollastonite in theinsulating material is 120-160 percent of the combined weights of thexonotlite and tobermorite.
 17. A method of making an insulatingmaterial, the method comprising: providing as dry solids 10-20 weightpercent previously-prepared xonotlite, 15-25 weight percent lime, 15-25weight percent silica, and 35-55 weight percent wollastonite; blendingthe dry solids with water to form a slurry; filter pressing the slurryto form a pressed shape; and curing the resulting pressed shape understeam pressure only until the resulting composition includes, inaddition to the wollastonite and any residual lime and silica, a calciumsilicate hydrate matrix having 51-90 weight percent xonotlite and 10-49weight percent tobermorite.
 18. The method of claim 17, comprisingcuring the resulting pressed shape under steam pressure only until theresulting composition includes, in addition to the wollastonite and anyresidual lime and silica, a calcium silicate hydrate matrix having 62-81weight percent xonotlite and 19-38 weight percent tobermorite.
 19. Themethod of claim 17, comprising curing the resulting pressed shape understeam pressure only until the resulting composition includes, inaddition to the wollastonite and any residual lime and silica, a calciumsilicate hydrate matrix having 66-76 weight percent xonotlite and 24-34weight percent tobermorite.
 20. The method of claim 17, wherein curingthe resulting pressed shape under steam pressure comprises curing theresulting pressed shape under saturated steam at 12-18 bar for 5 to 8hours.